When building a bike, it's very tempting to cut the pieces
and weld them together by eyeball. I have done this before and it does
work and the bike feels fine, but the bike usually dog-tracks a little due
to slight mis-alignments, and people
riding behind you will give you a hard time because your wheels are not
lined up with each other. This may cause both performance and ego to suffer to some degree. What's the answer? You need a
jig. Everyone needs a jig. A jig holds all the frame parts and the wheels
dropouts in alignment while you weld them together. This not only assures
you that the bike is actually straight when you are done, it also holds
all the frame tubes together which makes it
easier to weld up.
In it's most simple form, a jig needs to
hold the rear dropouts and front dropouts in alignment. If it can do that
you can build the craziest frame in the world between them and it will
still go straight down the road. It's better though, to also hold the
major frame components in place. Jigs can be made out of metal or wood.
Metal is better because you can make it more precise, but it is also more
difficult to build. Here's
a jig for an upright bike to give you an idea of what one a
professional one looks like.
Building a Recumbent Bike Frame Jig
I'll discuss two methods for building two bike frame jigs, first one made from
wood, then one made from metal,
and finally one that I built.
A wood jig is easy to build with a minimal number of tools, the metal jig
is harder but lasts longer, is more precise, and won't catch on fire if
you get carried away with the torch...
Wood Jig
Materials
- 1 - 4x8 sheet of 3/4" MDF (flat!)
- 1 - 2x4 (straight!).
- Phillips head wood screws
- Recommended tools include circular saw,
jigsaw, drill press, large straight edge, large square.
- 8 - 3/8" x 3" bolts and nuts
Wood Jig Platform
To build your wood jig you will first need a completely flat wood base. I
suggest starting with a 4x8 sheet of 3/4" MDF (medium density
fiberboard). This is better than plywood because the surface is really
flat, and it doesn't warp like plywood. All parts for this jig should be
made as accurately as possible as any inaccuracies here will be multiplied
by the inaccuracies made when building the bike parts!
- Cut a 2 feet wide by 8 ft long strip.
This is the base for your jig.
- Cut 2 - 6" wide strips and screw
them into the bottom 1.5 feet apart to reinforce the structure and
make sure it stays flat. Pre-drill and counter sink your holes to
prevent splintering.
- Cut another 6" strip into 1.5 foot
strips to go between the reinforcement strips. One at each end and 3
in the middle should be plenty.
- Mark a centerline down the middle of
your new jig platform in something permanent like magic marker.
- Drill a series of holes 3/8" holes,
2" apart exactly down the centerline. These holes will allow you to
vary your wheelbase length in increments of 2 inches.
Jig platform bottom view showing reinforcement
Dropout Standoff Jig
Now you have a really heavy but flat and
straight platform that you can put it on some saw horses when using it or
lean up in the corner when you're not. The next step is making the
standoffs for the dropouts. These will be made with sides of MDF and a
core of 2x4. Before building these you will need to figure out whether you
are using MTB or road hubs, and what size wheels and tires you will be
using. MTB hubs are 135mm spacing for rear wheels, road hubs are 130mm for
rear wheels and front wheels are usually 100mm. Cut a precise 2 1/4" strip from
the 2x4. Draw center lines on the center of the 2 1/4" width, both sides. This will be
the core of the standoff.
Dropout Standoff Jig
Cut out the sides of the
dropout standoff jig as shown in the
drawing. Make sure the bottom of the standoff is flat and square by putting it on the jig
platform and using a square to make sure it is standing up straight. Fasten
the sandwich together with wood screws. You will need to use either the axles
from old hubs (quick release axles work best) or some 3/8 threaded rod to make
your standoff axles. Measure the radius of your wheel and drill an axle hole
through the standoff at that diameter. Use a drill press to make sure the hole
is perpendicular to the side of the standoff. Drill 3/8" holes through the
centerline the base of the standoff to mount it to the holes in the jig platform (See picture).
Mount the axle in the axle hole using the thin nuts and washers to space it to
the correct width. Verify that the axle is centered properly on the standoff.
Use the 3/8" x 3" bolts to fasten the standoff to the holes in the
jig platform. Check that it's
square to the base. Check that the axle is the same distance to the base
on both sides. Check that the distance between the front and back dropout
axles is the same on both sides.
Frame Jig
Ok, now you have the base and a way to mount your fork
and rear dropouts in a solid way and make sure they line up properly. The next
step is making brackets to hold the frame together while you are welding it up.
For the sake of expediency we will be assuming you are building a
traditional monotube recumbent.
The frame jigs are constructed
in a similar manner to the Dropout standoff Jigs, except that the 2x4 does
not need to be cut down. There are two ways of securing the tubes. Type 1
involves using a hole saw of the same diameter of your frame tube. Cut the
hole in half and use wood screws to clamp the tube. Type 2 can be used for a
range of tubing sizes and involves cutting a precision V in the 2x4, then
using a metal strap to hold the tube in place. Draw a line down the center
of the 2x4 on both sides of the wide face to help center the frame tube hole
or V cut. By using a half circle for the side plate it is possible to adjust
the frame clamp standoff to a wide range of angles. You may need two frame
jigs per tube to support it securely. Side plates can be screwed to the
frame support 2x4, or clamped with C-clamps to provide easy adjustability.
Examples:
Here is a not-to-scale example showing how the jigs are used to hold
frame tubes together for welding.
This next drawing shows how the
dropout standoff jigs are used to hold the bike in alignment while the bikes
rear suspended suspension geometry is welded into place.
The wood frame jig in this
document is not meant to be the ultimate tool for frame construction, but
it's an easy way to make sure your bike turns out straight. If you are
planning on making more than a couple bikes, a custom bike jig made from
metal is a good idea.
Metal
Frame Jig
As with any good jig, you need to start off with a straight and flat
plane of reference. In this case we will be using 2x4" rectangular
steel tubing. This is a fairly common item that should be available at any
metal supply retailer. I suggest making your frame jig 8ft long. The cuts
should be precise so use a chop saw or have your local machine shop make the
cuts for you. You cab build the jig from rectangular steel tubing as in the
example below, or if weight is an issue (and money isn't!) you can build it
from 2x4 aluminum tubing and angle material.
Metal Jig Materials
-
30 ft of 2x4" 1/8"
wall steel tubing.
-
2 ft of 1.5" wide angle iron
-
8 - "C" clamps
with at least a 6.5" throat.
-
Steel chop saw, Welder or Brazing torch,
drill press, square, tape rule, big hammer, etc.
Bottom Bracket Jig
The final jig to be created is the bottom bracket jig. While a jig
using cones to properly center the BB would be preferable, making them
is probably outside most of our range of skill and budget, so we'll
just cheat a little.
Find yourself a junker 10 speed bike,
and remove the bottom bracket. Throw the bike away and save the BB's
threaded bearing cups. Make two 1/8" thick disks the same
diameter as the outside of the cups. Use a compass to make sure they
are round. Drill an 1/8" hole in the center to mark the exact
center. Braise the disks onto the outside of the cups. Take two more
4" C-clamps and mark the exact center of the moveable end of the
clamp. Cut the clamps in half and discard the non-adjustable side.
Drill a shallow 1/8" hole in the center of the moveable end of
the clamp. Mount the C-clamp to the bearing cup and use the shaft of
an 1/8" drill to verify alignment. |
Bottom Bracket Jig |
Braise the C-clamp to the
bearing cup. The bearing cup should be able to flop around on the end
of the C-clamp. Go find the bike you discarded and surgically remove
the entire BB. Screw the new C-clamps with BB cups into the BB. mark
the center of the BB and align it with the center of the jig standoff.
Mark the cut ends of the C clamps so they can be braised to the sides
of the standoff. Verify that the BB and clamps are aligned level and
straight before brazing them to the standoff. The BB alignment jig
will be a bit hard to put on and take off of a BB shell with the
threads, also there is a chance that that heating the BB shell will
cause the BB cup threads to fuse to the BB shell. To prevent these
issues you can carefully file off the threads on the BB cups until
they slide into the BB shell without having to screw them in. It
should be a tight fit but not so tight that it could damage the
threads.
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Completed Jig
Here's Larry Lem's completed
jig, holding his latest creation, a dual 700C Bacchetta clone.
Pictured below is the metal jig base,
with dropout and frame jigs. The frame and dropout jigs can be clamped into place between
the rails with C-clamps, allowing maximum adjustability.
Alternatively, a series of holes can be drilled through both
sides of the rails to allow the jigs
to be clamped using long bolts.
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Metal frame jig
Frame jigs can be inserted and
clamped at whatever angle is needed to build the components of your frame.
Steel construction insures alignment won't burn when you are brazing on your
rear dropouts.
Jason
Erickson built a frame jig from these plans
Warren's
Jig
As with many of my
projects, what gets built is not exactly the same as the original design. I
came across some old 7 foot tall computer racks (free!) this past winter
which looked exactly like what I needed to make a jig out of. I cut up the
bottom mounting plate for the end plates and "L" brackets, and
used the bolts that come with it to bolt it all back together a bit
differently. I built the jigs as in the metal frame jig above, and had
to pay $50 for the 8 foot chunk of 1-1/4" x 2" steel tubing (arg!).
The jig rails are a heavy
duty "U" shaped hunk of steel. I use those clamps to hold
the jig in place. I'm using hose clamps to hold the tube to the jig
for now, I may get ambitious later and cut up some C-clamps later, but
the hose clamps work fine for now.
I used more old rack part to make some
legs so I could actually braise standing up (what a concept!). In the
jig is the front sub-frame to the Cuda-W
speedbike I'm working on.
As shown in this article, a wide
variety of materials can be used to make a frame jig. The important
thing to remember with each of them, is that your bike will only be as
straight as the jigs your build, so measure carefully! |
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11/11/09
Steve Delaire sent this picture of his fork jig:
- adjustable offset 0 to 150mm
- axle holder is step threaded to
accommodate different axle diameters
- set screw tabs at axle to precision
align perpendicularity
- default width is 100mm ~ spacers
added for drive widths
- pic should explain the rest
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